1. Field of the Invention
The present invention is directed to a continuous method for removing oil vapor from a feed gas containing water vapor. More particularly, the present invention is directed to an improved adsorption method for the continuous removal of oil vapor from gas containing water vapor which comprises sequentially passing the feed gas through a first adsorption layer comprising a regenerable desiccant, a second adsorption layer comprising an oil adsorbent, and a third adsorption layer comprising a regenerable desiccant, and reversing the flow direction after a certain amount of time.
2. Description of the Prior Art
The removal of oil vapor from gaseous streams is essential for the preparation of many gaseous products. Examples of such oil-free gas products include oil-free air for breathing purposes, oil-free feed mixtures to prevent degradation of downstream processes such as permeable membrane separations, and oil-free gases to prevent oil vapor contamination of high purity products produced in other downstream processes.
Permeable membrane separation is a well known method for separating gaseous mixtures. Permeable membrane separation involves passing a gaseous feed mixture at an elevated pressure through a permeable membrane system to recover the more readily permeable component of the feed mixture as the permeate at low pressure and to recover the less readily permeable component of the feed stream as the residue stream at essentially the feed pressure. The term "gaseous mixture", as used herein, refers to a gaseous mixture, such as air, primarily comprised of two components having different permeabilities through the membrane material.
A major application of permeable membrane separations is in the field of air separation. Polymeric membranes are generally employed for this application. In most cases, oxygen is the more permeable component and becomes the enriched component in the permeate stream while nitrogen is the less permeable component and becomes the enriched component in the residue stream.
The permeable membrane separation systems must exhibit reasonable stability and must not suffer undue degradation during the separation method. In order to minimize cost, oil-flooded screw compressors are typically utilized to supply pressurized feed gas to the surface of a membrane separation system. Such oil-flooded compressors are also employed in other types of feed gas applications and other types of oil lubricated compressors are employed to compress feed gas to membrane separation systems. In such applications, the compressor generally contaminates the feed gas with oil which degrades the performance of the membrane system or otherwise has a detrimental effect upon the feed gas application. The presence of even relatively low concentrations of heavy hydrocarbon oil vapors, e.g., less than about 1 ppm by volume, can result in rapid and extensive loss of membrane permeability. Contaminants commonly present in ambient air, such as light hydrocarbons, water, and carbon dioxide, generally result in only a modest or no decrease in membrane permeability.
Because of the potential loss of membrane performance, membranes are generally sized with a safety factor sufficiently large to compensate for the anticipated permeability loss from all sources. However, neither over-design of the membrane system nor interruption of the gas product operation to renew the membrane is a satisfactory means for overcoming membrane permeability degradation.
One approach to preserve the permeability of a membrane is to provide a purification vessel containing an adsorption layer or trap to remove oil contaminants. The bed size of the oil adsorption layer is determined by the anticipated hydrocarbon loading of the adsorption layer and the contamination level of the feed gas stream being treated. If the quantity of oil adsorption layer is excessive, the adsorption vessel cost and the feed gas pressure drop across the vessel will be unnecessarily high resulting in higher power consumption costs. If the quantity of adsorption layer material is not sufficient, premature breakthrough of hydrocarbon vapors from the adsorption layer will take place and loss of membrane performance will occur.
Membrane separation systems are more fully described in "Membranes in Separations" by Hwang and Kammermeyer, Chapter XIII, Wiley (1975), U.S. Pat. No. 4,230,463, issued to Henis et al., and in U.S. Pat. No. 4,772,392, issued to Sanders, Jr. et al., which references are incorporated herein by reference.
A problem with using oil adsorption layers, such as activated carbon, to remove oil vapors from a feed gas is that such adsorbents also adsorb substantial amounts of water, especially at high relative humidity. When such oil adsorbents become saturated with water, the adsorbent is able to adsorb less oil requiring frequent changing of the oil adsorption layer.
One method for removing water vapor from a feed gas, prior to contacting the feed gas with an oil adsorption layer, involves contacting the gas with a hygroscopic agent, such as silica gel, molecular sieves, quick lime, calcium chloride, phosphorous pentoxide, lithium chloride, or concentrated sulfuric acid. This method has the advantage of being able to reduce the water vapor concentration in the feed gas to low levels but has the disadvantage of requiring an interruption in the purification process to dispose of, or regenerate, the used hygroscopic agent.
Another method for removing water vapor from a feed gas involves condensing the water vapor in the gas by compressing the gas and cooling the gas to below ambient temperature in a refrigerated cooler and then reheating the gas to above room temperature. This method has the advantage of being able to continuously remove water vapor in large scale but has the disadvantage of not being able to reduce the water vapor concentration to low levels and requiring a large quantity of energy and high maintenance.
Calculated and experimental adsorption equilibria values have been compared for adsorption of mixtures of water vapor and solvent on activated carbon adsorption layers, by Ozaki et al., J. Chem. Eng. Japan, Vol. 11, pp. 209-211 (1978). Ozaki et al. show in FIGS. 2 and 3 that at low relative humidity (below about 30%) activated carbon adsorbs little moisture while at high relative humidity, activated carbon adsorbs substantial amounts of moisture. At low relative humidity, adsorption of water occurs primarily by weak interactions between the water molecules and the activated carbon. At high relative humidity, hydrogen bonding causes the water molecules to cluster in the pores of the activated carbon causing the adsorbent to adsorb substantial amounts of moisture, up to 40% by weight. This adsorption of substantial amounts of moisture can be expected to reduce the oil-vapor adsorption capacity of the adsorbent substantially. The detrimental effect of oil vapor on permeable membrane performance is well known.
U.S. Pat. No. 4,783,201, issued to Rice et al., discloses a gas dehydration process which comprises contacting a feed gas containing water vapor with one side of an uncoated, asymmetric membrane having controlled porosity, permeating a majority of the water vapor in the feed gas through the membrane, and removing the resulting nonpermeate dehydrated gas from the membrane. Rice et al. show in Example 5 that membrane permeability, P/l in Table 1, drops by more than 50% when oil vapor is applied continuously to the membrane. This loss of permeability is irreversible.
Romano et al., "Proceedings of the Seventh Annual Membrane Technology/Planning Conference", pp. 168-169, Cambridge, Mass. (1989), describe the effect of oil vapors on the performance of permeable membranes. Romano et al. state that as much as a 25% decline in oxygen permeability of the permeable membrane with time occurs during ambient temperature operation.
U.S. Pat. No. 4,859,215, to Langsam et al., discloses a polymeric membrane for gas separation which comprises a silyl substituted polyacetylene polymer to which has been added an additive to increase the gas selectivity of the membrane. Langsam et al. state that the oxygen permeability of PTMSP (poly-trimethylsilyl 1-propyne) membranes decreases with exposure to oil vapors.
U.S. Pat. No. 3,672,824, to Tamura et al., discloses a method for removing carbon monoxide from moisture containing air which comprises oxidizing the carbon monoxide with a catalyst disposed between two dehydration chambers packed with a desiccant. The desiccant is capable of allowing adsorbed moisture to be desorbed. The method comprises passing the moist air containing carbon monoxide sequentially through the three adjacent chambers first in one direction until the first dehydration chamber becomes saturated with water then passing the air in the other direction for a similar length of time continuously and sequentially.
U.S. Pat. No. 4,881,953, to Prasad et al., discloses a process for purifying a gaseous mixture prior to separation of the mixture by passing the gaseous mixture through a bed of adsorbent material to adsorb the heavier hydrocarbon contaminants (greater than C.sub.5) responsible for degradation of the membrane without removing the lighter hydrocarbons. The compressed air is sent to an aftercooler, a moisture separator, and is then heated to prevent condensation in the adsorbent bed and in the membrane separation unit. The relative humidity of the feed gas entering the carbon bed is about 85%.
While the above methods for purifying a feed gas in a permeable membrane separation system provide improvements in the performance of such permeable membrane separation systems, none of these methods are entirely satisfactory. Common problems with conventional purification methods include high adsorption of water vapor and low adsorption of oil vapor in the oil adsorbent, frequent renewal of the oil adsorption layer and the permeable membrane system, the need to heat the gas entering the carbon bed to prevent condensation, and low adsorption of lower hydrocarbons (C.sub.5 and lower) in the oil adsorption layer resulting in contamination of the purified gas product. While the adsorption of large amounts of water vapor by oil vapor adsorbents is known, such water vapor adsorption reduces the oil vapor adsorption capacity of the oil vapor adsorbent. None of the above methods solves the problem of protecting the oil adsorption layer from moisture in an efficient manner and for extended periods of time. Hence there is a need for an improved method for continuously purifying a feed gas in a membrane separation system which can be employed economically. The present invention provides such an improved method for continuously and economically removing oil vapor from a feed gas containing water vapor in a membrane separation system.